1. Field of the Invention
The present invention relates to a mold clamping force setting device and a mold clamping force setting method of an injection molding machine for obtaining appropriate mold clamping force that can prevent burr generation due to the resin pressure when resin is injected into a mold of the injection molding machine and at the same time prevent deformation of the mold.
2. Description of the Related Art
In an injection molding cycle, molten resin is injected into a mold after the mold is closed in a mold closing process and then a mold clamping force is generated in a mold clamping process. When the molten resin is injected into the mold, burr may be generated with the mold opening if the mold clamping force is not sufficient with respect to the pressure with which the molten resin is generated. By contrast, if the mold clamping force is excessive, although there is no risk of burr generation, problems may occur that the product quality is lowered due to poor outgassing and the mold life is shortened due to excess mold clamping force applied to the mold. For this reason, the mold clamping force is set to the minimum value that does not open the mold at the time of injection, whereby burr generation is prevented, good outgassing conditions are enabled, and excess burden on the mold can be prevented. It is thus desirable that the minimum mold clamping force that does not open the mold be obtained and set.
As a technique to obtain the minimum mold clamping force, Japanese Patent Application Laid-Open No. 2008-6651 discloses a mold clamping force setting method with which the mold clamping force is gradually changed and the minimum mold clamping force to be applied to the mold is obtained based on the detected change in the mold clamping force. Furthermore, Japanese Patent Application Laid-Open No. 8-252849 discloses a mold clamping force setting method with which the minimum mold clamping force is obtained in view of change in the approximate linear relation established between the set mold clamping force and the maximum mold clamping force increase amount which is the difference between the maximum mold clamping force value during injection and the set mold clamping force.
There are problems in terms of effectiveness and accuracy with the mold clamping force setting methods described above, with which the minimum mold clamping force is obtained. When these mold clamping force setting methods are used in practical cases, it is necessary to identify the part of the mold that first opens and detect the mold clamping force applied on the area surrounding the part as an index.
A technique that solves the problems described above is disclosed in Japanese Patent Application Laid-Open No. 2013-75382 (corresponding to DE 10 2012 018 749 A1). According to this technique, the difference between the mold clamping force generated when the mold is closed and the mold clamping force during injection is detected as the maximum mold clamping force increase amount. From the detected maximum mold clamping force increase amount, the minimum mold clamping force that does not open the mold is obtained. With this technique, burr generation due to the mold opening can be prevented and injection molding performed under the condition where outgassing is easy can improve the product quality. Furthermore, the technique does not cause extra burden on the mold, thereby preventing the mold life from being needlessly shortened.
FIGS. 3A to 3C are diagrams illustrating the set mold clamping forces and the mold states during injection (see Japanese Patent Application Laid-Open No. 2013-75382). FIGS. 4A to 4C illustrate the mold starting to open from the upper part thereof during injection. The mold state as illustrated in State 3 in FIG. 4C is herein referred to as “a state in which the part of the mold that first opens is an upper part thereof”. In this state, the upper tie bar is extended to be longer than that at the mold clamping completion time whereas the length of the lower tie bar is kept approximately the same as that at the mold clamping completion time. To detect the mold clamping force, a mold clamping force detector is often provided on one tie bar out of a plurality of tie bars for detecting the tie bar extension. If the mold clamping force detector is provided on the upper tie bar when the mold is as in State 3 in FIGS. 4A to 4C, an increase in the mold clamping force, that is, opening of the mold can be detected after the mold is closed during injection. By contrast, if the mold clamping force detector is provided on the lower tie bar, the mold clamping force detector on the lower tie bar cannot detect change in the mold clamping force during injection, that is, opening of the mold.
In the part of the mold that is opening, the mold opening force by the resin pressure exceeds the mold clamping force applied on the mold. The opening part is decided depending on multiple factors such as distribution of forces that the mold receives from the molten resin and the mold clamping force balance. The distribution of forces that the mold receives from the molten resin depends on the shape and the arrangement of a cavity. Estimation of the distribution of forces may be easy in some cases, and difficult in other cases. The mold clamping force balance is hard to accurately grasp depending on the adjustment condition of the injection molding machine and the parallelization degree of parting faces of the mold, and thus has to be measured each time.
As described above, it is hard to estimate the part of the mold that first opens. This is why injection molding is actually performed to estimate the part of the mold that first opens based on the position where burr is generated. Furthermore, if the part of the mold that first opens is identified, a mold clamping force detector has to be provided on a tie bar nearest to the part to detect the mold clamping force applied on the area surrounding the part. The part of the mold that first opens is different for each mold. This may cause the burden of rearranging the mold clamping force detector on another tie bar in association with replacement of the mold.